Fracturing earth formations



United States Patent 3,332,491 FRACTURING EARTH FGRMATIUNS Don H. Flickinger, Tulsa, Okla, assiguor to Pan American Petroleum Corporation, Tulsa, Okla, a corporation of Delaware No Drawing. Filed July 28, 1965, Ser. No. 475,592 7 Claims. (Cl. 166-42) This invention relates to fracturing formations penetrated by wells where the formations are deep below the earths surface. More particularly the invention relates to controlling screenouts during such operations.

In deep wells the pressure required to initiate a fracture in a formation often exceeds the safe working pressure for the casing in the well. This is particularly true if the Well is more than about 10,000 feet in depth. In such cases a tubing string can be run into the well, a packer can be set between the tubing and the well wall near the bottom of the tubing string and the fracture can then be initiated by applying pressure through the tubing which can withstand the higher pressure.

Once a fracture is initiated, a valve incorporated in the tubing string above the packer can be opened so fracturing liquid can be pumped down both the tubing and annular space between the tubing and casing to extend the fracture. The valve may be considered a form of crossover valve permitting liquids from the annular space to enter the tubing. This is shown in U.S. Patent 2,716,454, Abendroth. A particularly advantageous valve for use in such cases is described and claimed in US. patent application S.N. 175,418 which I filed on Feb. 26, 1962 and now Patent No. 3,244,234.

The system works well even in deep wells until a screenout occurs. A screenout usually occurs when the fracture fills up with props causing the props in the fracturing liquid to deposit in the well or when the props form a bridge across the face of the fracture so props are deposited in the well. The result, in either case, is an excessive increase in pressure required to pump the fracturing liquid down the well.

In a deep well, the friction pressure drop due to flow of liquids in the tubular goods may already bring the pressure at the top of the well to a value close to the safe working pressure of the casing. Thus, even a small slow rise in pressure may be serious. In deep wells the problem is further aggravated by the high flow rates required to keep the fracture open; The bottom hole pressure required to open and extend a fracture, even after initiation, is approximately 0.7 or 0.8 pound per square inch per foot of depth. Pressures of liquids in the formations in areas Where the rocks are sufficiently well consolidated to fracture, rarely exceed around 0.5 pound per square inch per foot of depth. Therefore, in deep Wells, a high differential pressure exists between liquids in the well and fracture, and liquids in the formation. The result is a very high leak-off rate of fracturing fluid through the fracture faces and into the formation. This, in turn, requires a high injection rate to extend the fracture to any particular desired distance from the well. High injection rates mean high pressure drops due to friction of the flowing liquids down the tubing and annular space with resulting high pressures at the surface.

In deep wells the screenout problem is also serious because of the large volume of fracturing liquid in the well. Even under best conditions of high flow rates, a small amount of props can settle out on top of the packer and tend to stick it. In case of a screenout, if the pumps are stopped, the large volume of props in the annular space can settle on the packer sticking it in the Well. Even if props-free liquid is circulated down the annular space and up the tubing in an effort to clear the "ice props out of the well, the rate of circulation may be so slow that a serious amount of prop settling can occur.

One method of decreasing or overcoming screcnouts is described in US. Patent 2,888,988, Clark. Clark shows the introduction of props-containing liquid down the annular space and props-free liquid down the tubing. This technique can result in all the difficulties listed above if a screenout occurs, in spite of the props-free liquid pumped down the tubing, if a packer is set between the tubing and easing. In addition, the volume of prop-free liquid which can be pumped down the tubing is too small to be effective in reducing screenouts in most cases. If the streams are reversed so the props are pumped down the tubing and the props-free liquid down the annular space, the props concentration in the tubing is so high that it can easily bridge a fracture when the stream is discharged near the fracture face as required by Clark. The high concentration of props in the tubing can also cause another difiiculty. If the tubing pumps are shut off, the high concentration of props continues to settle into the bottom of the well at a rate which may increase the difficulties when the well already contains a considerable amount of props.

Because the injection rate must be so high in deep wells, any reduction in the volume of injected fracturing liquid may cause the fracture to close at least partly. Thus, if a screenout occurs, and the stream containing the props is decreased or stopped, the total volume of injected liquid decreases causing the fracture to tend to close. This may aggravate the screenout rather than relieve it.

Due to the high pressure and flow rate at the bottom of a deep well during fracturing operations, it is easy to overflush the fracture with prop-free liquid leaving the critical zone near the well free from props. It is also easy to allow the well to become unbalanced so that back flow occurs washing at least some of the props from the critical zone near the well before the fracture can close and effectively hold the props in place.

With all these problems in mind, it is an object of this invention to fracture a formation deep below the earths surface from a well penetrating the formation. A more specific object is to provide a method of fracturing deep formations which is capable of avoiding, decreasing, or overcoming screenouts. A still more specific object of the invention is to provide a method for completing a fracturing operation which avoids the danger of flushing the props out into the fracture away from the zone near the well and also avoids the danger of back flowing the props out of this zone near the well before the fracture can close and hold the props in place.

In general, I accomplish the objects of my invention by using a modified combination of the methods of Abendroth and Clark. By pumping props-free liquid down the annular space and props-containing liquid down the tubing, by maintaining the ratio of props-free to propscontaining liquid between the limits of about 2 to l and about 4 to l, and by mixing the two streams at least about 50 feet above the fracture, most of the screenout difficulties can be overcome.

This method keeps props out of the annular space so the packer is not stuck. I have found that by maintaining a props-free to props-containing stream ratio of at least about 2 to l, stopping the props-containing stream usually does not aggravate the screenout. What this really means, apparently, is that a fracture will accept a propsfree stream at /3 the rate at which it will accept a propscontaining stream. Of course there are some variations, depending upon circumstances, but the 2 to 1 minimum ratio is a safe one in nearly all cases. If the ratio is not more than about 4 to l the props concentration need not Dill-Tbi \Jutrn exceed an amount which can be pumped through the tubing. When reference is made to size or flow rates of the two streams, these are in terms of volume per unit time.

I have also found that by mixing the two streams at least about 50 feet above the fracture, sufiicient mixing is provided to avoid local concentrations sufiicient to bridge the fracture. The tubing may end a short distance below the packer or may extend on down the well to approximately the level of the fracture. While extending the tubing on down the well decreases mixing time, the greater turbulence caused by the higher velocity tends to compensate for the shorter mixing time. Therefore, it seems to make little difference whether the mixed streams flow at least 50 feet down the tubing or down the well below the bottom of the tubing.

In case of serious screenouts, it is usually advisable not only to stop pumping down the tubing, but to open a choke valve at the top of the tubing so at least a small rate of flow up the tubing can be maintained. This flow tends to lift the props up the tubing as fast as they settle down through the liquid in the tubing so no props leave the bot-tom of the tubing. An upward How of about gallons of water per minute in 2% tubing, for example, will prevent sand, glass beads, and aluminum props from dropping out the bottom of the tubing. A slightly higher rate of flow to provide a safety factor is generally preferred. A small meter on the choke valve line is advisable for the purpose of accurately measuring and controlling flow. The meter is also required in a preferred method for completing the fracturing operation.

In this method for completing the fracturing operation, as soon as the last of the props are introduced into the top of the tubing, nearly enough props-free liquid is pumped into the tubing to displace the props out of the bottom of the tubing. In order to avoid overfiushing the fracture and washing the props back into th fracture away from the critical zone near the well, the tubing pump is stopped while a small amount of props remain in the tubing. Then the choke valve at the top of the tubing string is opened and props-free liquid is pumped down the annular space and up the tubing at a rate which will lift the remaining props up the tubing and clear them out of the well. About two or three barrels per minute is usually a good fiow rate for this purpose. During this operation a meter on the input line and the meter on the choke valve line are closely observed. The choke valve is controlled so the volume of liquid flowing out of the well is substantially equal to the volume pumped into the well. Under these conditions, little liquid can enter or leave the fracture so no displacement of the props in the fracture can occur. As soon as the props are all washed out of the tubing, the well is shut-in to permit the fracture to close and hold the props in place. It will be apparent that this method of balanced fiows can be used at any time during the fracturing operation to avoid removal of props from the portion of the fracture near the well.

The props can be any of the well known propping materials known in the art. These include, sand, glass beads, metal pellets, ground nutshells, plastics, and the like. These may be mixed with temporary particles such as oil soluble resins or Water soluble pellets such as urea prills which will dissolve in formation liquids in the fracture and leave the permanent props spaced apart. Prop sizes may also be any of those now used. In general, they may vary from those barely retained on a number 60 U.S. Standard sieve, to those barely passing a number 4 sieve. The size range best adapted for use in any particular field is ordinarily well known in that field.

The fracturing liquid can also be any of those ordinarily used. These may include oil, water, acids or emulsions incorporating these liquids. The liquids may include additives such as soaps in the oil or gums in the water or acid to increase viscosity, decrease filtration rate, decrease friction pressure drops and the like. The water phase may include acids such as hydrochloric acid if desired.

The liquid in the tubing string may be different from that in the annular space. For example, the pressure drop in the tubing is usually greater than that in the annular space. Therefore, it may be advisable to use friction reducers in the tubing liquid, while omitting them from liquid in the annular space where they are known to have less advantage. On the other hand, if a viscosity increasing additive is used, it is generally best to add it only to the liquid in the annular space and omit it from liquid in the tubing. This is particularly true since the liquid in the tubing, containing the high concentration of props, already produces high pressure drops in the tubing. The liquid in the tubing may be oil, while that in the annular space is water, an emulsifying agent being included in one or both streams so a viscous emulsion will be formed at the bottom of the well while non-viscous liquids are pumped down both the tubing and annular space. Many other variations will be apparent to those skilled in the art.

The liquid in the annular space should be substantially solids-free. As noted above, however, this liquid may contain solids such as soaps in colloidal dispersion. Actually solid particles up to those barely passing a number U.S. Standard sieve cause little difiiculty as far as screenouts are concerned, so the term substantially solidsfree is not intended to exclude the presence of such small particles. This is particularly true since the amount of colloids or other small particles used in fracturing fluids usually is not much more than 1 percent by weight and frequently is considerably less. Even up to about 0.1 pound of particles in the size range of fracture props can be tolerated per gallon of liquid in the annular space without greatly affecting screenouts. Therefore, the term substantially solids-free should be interpreted to include prop size particles in an amount up to about 0.1 pound per gallon. Preferably, however, the liquid pumped down the annular space should be entirely free of solids above the colloidal range.

The concentration of props ordinarily used in fracturing liquids ranges from about to about 5 pounds per gallon and preferably from about to about 1 pound per gallon. The concentration of props in the liquid in the tubing must, of course, be higher. With a large casing and small tubing, the rate of fracturing fluid flowing down the annular space may be as much as about 4 times that flowing down the tubing. If a final concentration of props in the mixed streams at the bottom of the well is to be on the order of 2 pounds per gallon in such cases, the concentration in the tubing must be about 10 pounds per gallon whereas if a final concentration of 0.5 pound per gallon is desired, the tubing concentration woudl be 2.5 pounds of props per gallon of liquid.

When reference is made to pumping or introducing a suspension of props down the tubing, it will be understood that one or more strings of tubing can be used. The term annular space is also to be interpreted broadly to include not only the ring-shaped space between a single string of tubing and the casing in a well, but also the more or less ring-shaped space around a m-ulti plicity of strings of tubing in the well.

It will be understood that when reference is made to introducing props-containing liquid down the tubing, this need not be done at all times during the fracturing operation. For example, the fracture may first 'be initiated and extended to some degree by pumping props-free liquid down the tubing at the beginning. After pumping props-containing liquid down the tubing, it may be desirable occasionally to introduce a batch of props-free liquid. At the end of the operation, as explained above, the last of the props-containing liquid is usually displaced down the tubing with props-free liquid.

When reference is made to hydraulic fracturing or formations, it will be understood that this operation may include not only initiating new fractures, but also extending old ones. Thus, my process can be applied to old wells previously subjected to hydraulic fracturing operations, as well as to wells in which the formations have not been previously fractured. The term also applies to either producing wells or to injection wells in secondary recovery systems.

In one method of fracturing either new or old wells, after one fracture has been extended to the desired degree and propped, it can be sealed at its face and a second fracture can be initiated, extended, or both initiated and extended. In the sealing process, particles of a temporary sealing agent such as naphthalene are suspended in a carrying liquid and transported to the fracture face. It will be apparent that such temporary particles can also be carried down the tubing string or strings while a substantially solids-free liquid is pumped down the annular space, the two streams being mixed at a level at least 50 feet above the fracture to form a suspension containing the desired uniform concentration of particles. After the existing propped fractures are sealed, pressure can then be applied to initiate or extend other fractures. If other fractures are to be initiated, it may, of course, be necessary to close the valve incorporated in the tubing string so high pressures can be applied through the tubing only.

My invention will be better understood from the following example. In a deep Well in Mississippi, inch casing was run to the bottom of the well and cemented in place. The casing was perforated over a foot interval at about 13,800 feet.

A string of 2% inch tubing was run with a packer and crossover valve set about 50 feet above the perforations. The valve was of a check valve type automatically preventing flow from the tubing to the annular space, but permitting flow from the annular space into the tubing. After setting the packer, the casing and tubing were filled with props-free water. The water in the tubing contained a friction-reducing agent. Pressure was applied to the liquid in the tubing to initiate a fracture.

After the fracture was initiated, high strength glass bead props were added to the Water pumped into the tubing. A little before the time the props reached the bottom of the tubing, water was pumped into the annular space. The two streams mixed at the valve, the mixed stream flowing on down the well and into the fracture. The concentration of props in the liquid pumped down the tubing was slowly increased from about /2- pound per gallon at the start, to about 2 /2 pounds per gallon at the end. The rate of flow down the tubing was about 5 barrels (42 US. gallons per barrel) per minute, while the rate down the annular space was about 13 barrels per minute. This provided a props concentration in the mixed streams which varied from about 0.14 to about 0.68 pound per gallon and a combined tubing and annulus rate of 18 to 19 barrels per minute.

There were no troubles with screenouts until near the end when the higher concentrations of props entered the fracture. Then a pressure build-up at the surface indicated a screenout was developing. Shutting down one of the two pumps introducing liquid down the tubing, did not cause the pressure to decrease to its former value, so the other tubing pump was also shut down. The screenout quickly cleared up and the two tubing pumps were started again. After a short time another screenout was indicated. This time the more customary procedure of shutting down all pumps and back flowing from the formation up the tubing, was employed. Again the screenout was cleared up and props-free liquid was pumped down the annular space and into the fracture. Shortly after the tubing pumps were started, however, a screenout again developed. It was concluded that the fracture had been extended as far as possibl with the pumps available, and

6 that the fracture had been filled with props. Therefore, it was decided to terminate the fracturing operation.

At this point, about the lower half of the tubing string contained props. These were removed by pumping and metering about two barrels per minute of water into the annular space and metering the same flow of water out the top of the tubing through a controlled choke valve until the props had been removed from the tubing. The well was then shut-in overnight to permit the fracture to close on the props.

Several variations and modifications of my process have been described above. These are by way of example only. Still other modifications and variations will be apparent to those skilled in the art. Therefore, I do not wish to be limited to the examples given, but only by the following claims.

I claim:

1. A method for fracturing a deep earth formation penetrated by a well comprising running a string of tubing into said well, said tubing having near its bottom end a valve which can be closed to flow of liquids from the tubing into the annular space between the tubing and well wall but can be opened for flow of liquid from said annular space into said tubing, said tubing string carrying below said valve a packer surrounding said tubing, setting said packer in said well at a level such that said valve is at least about feet above the level of the fracture, introducing down said tubing string a first stream which is a suspension of solid particles in a carrying liquid, at the same time introducing down said annular space a second stream which is substantially free from solid particles retained on a number 100 US. Standard sieve, the rate of flow of said second stream being from about 2 to about 4 times the rate of flow of said first stream, both rates of flow being in terms of volume per unit time, and applying to said streams a pressure sufficient to extend a fracture in said formation.

2. The method of claim 1 in which said solid particles include permanent props for the fracture.

3. The method of claim 2 in which props are removed from the tubing by introducing substantially solids-free liquid down the annular space and up the tubing to lift the remaining particles up and out of said tubing while controlling the volume per unit time flowing out of said tubing to make it substantially the same as the volume per unit time flowing into the annular space so there is substantially no flow into or out of the fracture and therefore no displacement of props back away from the well in the fracture or flow of props out of the fracture into the well while lifting the props out of the tubing.

4. The method of claim 2 in which at the end of the operation the props-containing liquid is displaced down the tubing with a substantially solids-free liquid, but flow down said tubing is stopped before all of the props are displaced out of the bottom of said tubing to avoid the possibility of overflushing the fracture, and after stopping flow down said tubing, opening the top of said tubing, introducing substantially solids-free liquid down the annular space and up the tubing to lift the remaining particles up and out of said tubing while controlling the volume per unit time flowing out of said tubing to make it substantially the same as the volume per unit time flowing into the annular space so there is substantially no flow into or out of the fracture and therefore no displacement of props back away from the well in the fracture or flow of props out of the fracture into the well while lifting the props out of the tubing.

5. A method for controlling screenouts during the hydraulic fracturing of a deep earth formation penetrated by a well in which a packer is run into the well on a tubing string and is set between the tubing string and the well wall and in which a valve is provided above said packer, which valve can be closed to prevent flow from the tubing into the annular space surrounding said tubing but can be opened for flow from said annular space into said tubing, comprising setting said packer at a level so said valve is at least about 50 feet above the level of the fracture, introducing down said tubing a first stream which is a concentrated suspension of fracture props in a carrying liquid, introducing down the annular space between the tubing and well wall a second stream which is a liquid substantially free from solid particles retained on a number 100 US. Standard sieve, the rate of flow of said second stream being from about 2 to about 4 times the rate of flow of said first stream, both rates of flow being in terms of volume per unit of time, mixing said first and second streams at said valve, applying sufiicient pressure to said streams to extend a fracture in said formation, injecting the mixture of streams into said fracture, and, when the pressure increases at the surface, indicating the formation of a screenout, decreasing the ratio of said first stream to said second stream to decrease said screenout.

6. The method of claim 5 in which the flow rate of said first stream is reduced to zero upon indication of the formation of a screenout, while flow of said second stream is continued.

7. The method of claim 6 in which, after stopping the flow of said first stream, the top of said tubing string is opened slightly to permit flow of liquid up said tubing string to prevent propping particles from settling down said tubing and entering the stream of substantially solidsfree liquid being pumped down the well and into the fracture.

References Cited UNITED STATES PATENTS 2,716,454 8/1955 Abendroth 16642 2,888,988 6/1959 Clark l66-42 X 3,224,506 12/1965 Huitt l6642 CHARLES E. OCONNELL, Primary Examiner.

N. C. BYERS, Assistant Examiner. 

5. A METHOD FOR CONTROLLING SCREENOUTS DURING THE HYDRAULIC FRACTURING OF A DEEP EARTH FORMATION PENETRATED BY A WELL IN WHICH A PACKER IS RUN INTO THE WELL ON A TUBING STRING AND IS SET BETWEEN THE TUBING STRING AND THE WELL WALL AND IN WHICH A VALVE IS PROVIDED ABOVE SAID PACKER, WHICH VALVE CAN BE CLOSED TO PREVENT FLOW FROM THE TUBING INTO THE ANNULAR SPACE SURROUNDING SAID TUBING BUT CAN BE OPENED FOR FLOW FROM SAID ANNULAR SPACE INTO SAID TUBING, COMPRISING SETTING SAID PACKER AT A LEVEL SO SAID VALVE IS AT LEAST ABOUT 50 FEET ABOVE THE LEVEL OF THE FRACTURE, INTRODUCING DOWN SAID TUBING A FIRST STREAM WHICH IS A CONCENTRATED SUSPENSION OF FRACTURE PROPS IN A CARRYING LIQUID, INTRODUCING DOWN THE ANNULAR SPACE BETWEEN THE TUBING AND WELL WALL A SECOND STREAM WHICH IS A LIQUID SUBSTANTIALLY FREE FROM SOLID PARTICLES RETAINED ON A NUMBER 100 U.S. STANDARD SIEVE, THE RATE OF FLOW OF SAID SECOND STREAM BEING FROM ABOUT 2 TO ABOUT 4 TIMES THE RATE OF FLOW OF SAID FIRST STREAM, BOTH RATES OF FLOW BEING IN TERMS OF VOLUME PER UNIT OF TIME, MIXING SAID FIRST AND SECOND STREAMS AT SAID VALVE, APPLYING SUFFICIENT PRESSURE TO SAID STREAMS TO EXTEND A FRACTURE IN SAID FORMATION, INJECTING THE MIXTURE OF STREAMS INTO SAID FRACTURE, AND, WHEN THE PRESSURE INCREASES AT THE SURFACE, INDICATING THE FORMATION OF A SCREENOUT, DECREASING THE RATIO OF SAID FIRST STREAM TO SAID SECOND STREAM TO DECREASE SAID SCREENOUT. 